In the perfect eye, an incoming beam of light is focused through the cornea and through the crystalline lens in a way that causes all of the light from a point source to converge at the same spot on the retina of the eye. This convergence occurs because all of the optical path lengths, for all light in the beam, are equal to each other. Stated differently, in the perfect eye, the time for all light to transit through the eye will be the same regardless of the particular path that is taken by the light.
Not all eyes, however, are perfect. The consequences of this are that light path lengths through the eye become distorted and are not all equal to each other. Thus, light from a point source that transits through an imperfect eye will not necessarily be focused on the retina, or to the same spot on the retina.
Normally, as light enters and passes through an eye it is refracted at the anterior surface of the cornea, at the posterior surface of the cornea, and at the surfaces of the crystalline lens. After all of these refractions have occurred, the light finally reaches the retina. As indicated above, in the case of the perfect eye, all of these refractions result in no overall change in the optical path lengths of light in the incoming beam. Therefore, any deviations resulting in unequal changes in these optical path lengths are indicative of imperfections in the eye that may need to be corrected.
In general, vision difficulties in the human eye can be characterized by the changes and differences in optical path lengths that occur as light transits through the eye. These difficulties are not uncommon. Indeed, nearly one half of the world's population suffers from imperfect visual perception. For example, many people are nearsighted because the distance between the lens and retina is too long (myopia). As a result, the sharp image of an object is generated not on the retina, but in front of or before the retina. Therefore, for a myopic person a distant scene appears to be more or less blurred. On the other hand, hyperopia is a condition wherein the error of refraction causes rays of light entering the eye parallel to the optic axis to be brought to a focus behind the retina. This happens because the distance between the lens and retina is too short. This condition is commonly referred to as farsightedness. Unlike the myopic person, a hyperopic, or farsighted, person will see a near scene as being more or less blurred.
Another refractive malady is astigmatism. Astigmatism, however, is different than either myopia or hyperopia in that it results from an unequal curvature of the refractive surfaces of the eye. With astigmatism, a ray of light is not sharply focused on the retina but is spread over a more or less diffuse area.
Further, in addition to the more simple refractive errors mentioned above, the human eye can also suffer from higher order refractive errors (“aberrations”) such as coma, trefoil and spherical aberration. More specifically, coma is an aberration in a lens or lens system whereby an off-axis point object is imaged as a small pear-shaped blob. Coma can be described as a wavefront shape with twofold symmetry and is caused when the power of the zones of the lens varies with distance of the zone from the axis. Likewise, trefoil is described as a wavefront shape having threefold symmetry. Spherical aberration results from loss of definition of images that are formed by optical systems, such as an eye. Such aberrations arise from the geometry of a spherical surface. For these higher order aberrations (“HOAs”), an ideally flat ‘wavefront’ (i.e. a condition wherein all optical path lengths are equal) is distorted by a real-world optical system. In some cases, these distortions occur in a very complex way. In the trivial case, non-higher order distortions like nearsightedness and farsightedness would result in an uncomplicated bowl-like symmetrical distortion. With HOAs, however, the result is a complex non-symmetrical distortion of the originally flat wavefront. It is these non-symmetrical distortions which are unique for every optical system (e.g., a person's eye), and which lead to blurred optical imaging of viewed scenes.
While a typical approach for improving the vision of a patient has been to perform refractive surgery on the eye to eliminate distortions, the surgery itself can lead to an increase in HOAs, both immediately and during recovery. Indeed, it has been determined that conditions such as biomechanical stress distribution and hydration levels can induce changes in the optical characteristics of an eye as a mere consequence of corneal dissection. Specifically, the creation of a flap in the cornea by a mechanical microkeratome can induce HOAs including vertical coma, horizontal coma, spherical aberration and 90/180° astigmatism. While inducing fewer HOAs, the use of femtosecond lasers to create a flap in the cornea has not led to the complete elimination of such changes.
In general, when a corneal flap is created for purposes of performing refractive surgery, HOAs can result from two distinctly different circumstances. For one, HOAs can be physically introduced during the actual creation of the flap. As mentioned above, these HOAs typically result from the redistribution of biomechanical stresses that occur in the cornea as the flap is being created. For another, HOAs also result from physical characteristics of the cornea, and from the eye itself. For example, it is known that decentration (i.e. a condition wherein the anatomical and optical axes of the eye are not properly aligned) will create HOAs. Regardless of their source, however, HOAs can be troublesome and, if possible, should be corrected.
In light of the above, it is an object of the present invention to provide a method and system that measures the topology of the cornea, or other transparent material, in order to predict the effect of the creation of a flap thereon on HOAs. Another object of the present invention is to provide a method and system that incorporates the anatomical conditions in the cornea into surgical planning to compensate for surgically induced changes in HOAs. Another object of the present invention is to provide a method and system that incorporates pre-flap-creation wavefront data into the dimensional planning of the flap. Yet another object of the present invention is to provide a method and system that uses a real-time, closed-loop, adaptive-optical control system to reposition the transparent material during flap creation to match the pre-flap-creation optical conditions. Still another object of the present invention is to provide a method and system for predicting and precompensating for changes in HOAs induced by flap creation which are effectively easy to use, relatively simple to operate and implement, and comparatively cost effective.